A growing number of U.S. communities have discovered per- and polyfluoroalkyl substances (PFASs, often called PFCs) in drinking water. Epidemiological studies have reported negative associations between pediatric PFAS exposure and immune function, and suggest that current drinking water guidelines may not adequately protect children from immunotoxicity. In 2016, the National Toxicology Program concluded that two PFASs, PFOA and PFOS, are presumed immune hazards based on human and animal studies. A key knowledge gap is whether exposure to drinking water contaminated by AFFF firefighting foams with complex PFAS mixtures causes clinically relevant immunotoxicity. Communities seek guidance for interpreting blood and water test results and information on health effects, water treatment, remediation, and strategies to engage local officials. This project will leverage existing researcher-community partnerships to address concerns about health effects from PFASs in drinking water and develop tools and materials to support impacted communities. We will engage with two communities with AFFF-contaminated drinking water, the Pease International Tradeport in Portsmouth, NH, and Hyannis, MA, and evaluate potential immune responses and explore metabolomics profiles among young children. We will develop an online resource center to serve PFAS-affected communities nationwide with educational materials, data tools, and opportunities to connect with other communities. We will explore the exposure experience of affected communities by conducting in-depth interviews and ethnographic research. Project partners include environmental health and social science researchers and community activists with a well-established collaboration. This community-engaged research project has three aims: Aim 1. Quantify associations of child serum PFOA, PFOS, PFHxS, and total PFASs with a) serum antibody levels following diphtheria and tetanus (DTaP) vaccinations and b) metabolomic profiles. Aim 2. Develop the PFAS Exchange, an online resource center with educational materials, data interpretation tools, tap water testing, and resources to connect communities, provide report-back materials for participants, and engage with residents of affected communities, medical professionals, and other stakeholders. Aim 3. Assess individual, family, and community-level experience of residents in areas impacted by PFAS- contaminated drinking water. The proposed study is innovative in developing novel web-based tools to visualize and interpret personal exposure data, and will be the first to evaluate immunotoxicity and apply novel metabolomics methods in U.S. children exposed to AFFF-contaminated drinking water in early life. It will improve public health by supporting efforts to reduce exposures and health impacts of PFAS contamination through engagement, education, and research. This project fulfills the Research to Action mission by addressing community concerns about health effects of contaminated drinking water and involving community members throughout.